Abstract

Speciation in vertebrates is often viewed as a three-stage process beginning with an allopatric phase (geographic isolation), followed by secondary contact, and finally the transition to coexistence in overlapping geographical ranges (sympatry). In some forms of this model, the delay in establishing secondary sympatry is due to the slow divergence in ecological traits, where such divergence reduces competition6 and/or reproductive interference. However, we know little about the general tempo and timing of ecological trait divergence for allopatrically speciating pairs, and how these factors impact transitions from allopatry to sympatry. Here, we combine divergence time estimates, trait measurements, and geographic range data for 952 avian sister species pairs worldwide to examine the tempo and timing of ecological trait divergence, and how such divergence may impact the three-stage speciation process. Our analyses indicate that sister pair divergences in body mass and beak morphology, important ecological traits, are better explained by a pulse-and-stasis evolution model than a gradual divergence model. For sister pairs in secondary contact, body mass divergence and beak divergence are associated with earlier transitions to sympatry. Our evidence suggests that the contribution of trait divergence to the transition to sympatry stems from pulses of trait divergence early in the speciation process, with a limited contribution from gradual trait divergence. Incorporating early trait divergence pulses of varying magnitude into the three-stage speciation model can explain a perplexing set of observations in bird speciation: prolonged mutual exclusion in some older species pairs, marked geographic divergence early in speciation, and instances of rapid sympatry.

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